1. Field of the Invention
The present invention relates in general to the combination of ablative cooling with convective cooling and relates in particular to cooling an inlet ramp for a scramjet engine using such combined cooling methods.
2. Description of Prior Developments
During the development of high speed or hypersonic aircraft, it is general practice to conduct actual in-flight testing of engines and propulsion systems in order to better measure and understand the performance of the propulsion system. In the case of missiles or aircraft powered by ramjet or scramjet engines, it is very difficult or impossible to achieve realistic test conditions during ground tests. For this reason, it is beneficial to boost the vehicle to a predetermined test altitude and hypersonic test velocity using a rocket. At this point, the rocket is turned off and the test engine is turned on for in-flight testing. Once testing is completed, the vehicle is returned to earth for evaluation.
During the ascent, testing and descent phases of flight, the inlet ramp to the scramjet engine is subjected to varying heat loads generated by high speed airflow. A typical scramjet engine inlet includes multiple ramped surfaces which are aligned at progressively steeper angles relative to the incoming airflow. As the incoming air impinges on these ramps, it is compressed by passing through successive shock waves.
This compression increases both the static pressure and temperature of the air adjacent the scramjet inlet. When this heated pressurized air is flowing at a very high speed, it transfers a large amount of heat to the inlet ramps. The amount of heat transferred to the inlet ramps increases as the air temperature and air pressure increase.
For the high speed air flows encountered during the various phases of in-flight scramjet tests, the scramjet engine inlet ramps must be cooled in order to prevent them from overheating and burning or melting away. Radiation cooling, film cooling, transpiration cooling, convection cooling and heat-sink cooling, among others, are all possible methods which may be applied to cool the inlet ramps. However, because of the significantly varying heat loads experienced during the different phases of in-flight testing, one cooling method alone does not appear capable of practically or efficiently handling the overall cooling task.
As a ramjet or scramjet flight test vehicle is boosted by rocket to its test altitude and velocity, it is desirable to close off the inlet to the engine. The inlet is preferably closed in order to reduce the total heat load on the engine by preventing hot high-speed air from entering the internal engine flowpath during boost.
Such inlet closing is carried out, for example, by pivoting outwardly one or more of the inlet ramp surfaces so as to block off the scramjet air inlet. Unfortunately, by so moving the inlet ramp surface, the incidence angle of the air on the pivoted or actuated ramp surface increases and thereby increases the local heat load on these pivoted surfaces.
Once a desired altitude and velocity has been achieved, the rocket booster is deactivated and the test phase of the flight begins. The inlet to the scramjet engine is then opened to allow air into the engine for combustion. This opening may be carried out by moving the ramp surfaces to a position where the entire inlet ramp defines a smooth flat ramped surface for guiding air into the inlet.
It is most important during operation of the scramjet that these ramp surfaces maintain a well controlled geometry in order to provide smooth airflow into the scramjet engine. In the event these surfaces are warped, melted or burned from overheating, either during the ascent or test phases of flight, airflow into the engine can be disrupted and engine performance can be adversely affected. Thus, it is important that the ramp cooling system maintain a smooth ramp surface during both ascent and test. Convection, film, transpiration, heat sink and radiation cooling systems all can maintain this smooth surface. Ablative cooling schemes generally cannot.
It is preferable to have clean airflow into the engine during operation with as little turbulence as possible. The cooling of the inlet ramp during scramjet operation should also avoid the introduction of foreign substances or contaminants into the inlet. The scramjet operation could be adversely affected by the injection and passage of coolant gases or particles through the engine.
Film, transpiration, and ablative cooling systems all introduce foreign matter into the flow system, so they are not desirable for use during the test phase of flight. Radiation and heat sink cooling do not appear capable of handling the high heat loads on the inlet ramp. Convection cooling appears to be most suitable for this application.
After a period of in-flight testing, the scramjet is deactivated or turned off to allow the test vehicle to decelerate and descend to ground level. During descent, in order to reduce the total heat load on the engine, it is important to prevent hot high-speed air from entering the scramjet through the engine inlet. This may be accomplished by the same procedure mentioned above, i.e. by moving a portion of the inlet ramp across the front of the inlet.
Again, the ramp must be cooled as it is heated by high velocity air during descent. Without cooling, the ramp which contacts the high-speed air will burn or melt away, thereby making evaluation of the flight test more difficult or impossible.
Although convective cooling could be used to cool the ramp during the boost and test phases, it is not well suited for cooling the ramp during the descent phase. The main reason is that convection cooling during the descent phase would require the use of a large quantity of coolant due to the high heat load and long heat exposure period prior to reaching ground level.
In fact, the heat loads experienced by the inlet ramp during descent are greater than those experienced during the boost and test phases of the flight. Since convective cooling in aircraft often uses liquid fuel as the coolant, excessive fuel would be required to handle the heat loads during descent. This is clearly not a weight efficient solution to the inlet ramp cooling problem during descent. For the same reason, film and transpiration cooling are not suitable for cooling during the descent phase.
Accordingly, a need exists for a method and apparatus for cooling an inlet ramp to a scramjet engine while it is being boosted to operational speed and altitude with a rocket.
A further need exists for a method and apparatus for cooling an inlet ramp to a scramjet engine as the scramjet is operational, such as during flight tests, while avoiding the introduction of coolant or contaminants into the inlet of the scramjet.
Another need exists for efficiently cooling the inlet ramp of a scramjet engine of a test vehicle as it descends to ground level.